Liquid crystal displays ("LCDs") of relatively small size have been commercially available for over two decades. Recent improvements have permitted development of large size, high resolution displays which are useful in notebook and desktop computers. These LCD panels, particularly color LCD panels, are also used for flat screen televisions, projection television systems, and camcorder view finders. Many more applications are anticipated in the future.
LCD panels are of two types: passive matrix LCDs and active matrix LCDs ("AMLCDs"). Passive matrix displays employ transparent electrodes patterned in perpendicular striped arrays on facing glass plates. Red, green, and blue color filters on the inner surface of one of the glass plates provide the full color display. The passive matrix display is thought to be easier to fabricate than AMLCDs but has much more limited performance capabilities.
The fabrication of AMLCDs involves several steps. In one step, the front glass panel is prepared. This involves deposition of a color filter element onto a suitable substrate, such as glass. Color filter deposition typically involves depositing a black matrix pattern and three primary (typically either red, green, and blue or yellow, magenta, and cyan) color dot or color cell patterns within the spaces outlined by the black matrix. The printed lines which form the black matrix typically are about 15-40 microns wide and about 0.5 to 2 microns thick. The color cells are typically on the order of about 70 to 100 microns in width by 200 to 300 microns in length. The color cells are typically printed in films less than about 10 microns thick, and preferably less than 5 microns thick, and must be evenly applied and accurately registered within the pattern formed by the black matrix. The front glass substrate is typically completed by depositing a planarizing layer, a transparent conducting layer, and a polyimide alignment layer over the color filter element. The transparent conducting layer is typically indium tin oxide ("ITO"), although other materials can also be utilized. In a second step, a separate (rear) glass panel is used for the formation of thin film transistors or diodes, as well as metal interconnect lines. Each transistor acts as an on-off switch for an individual color pixel in the display panel. The third and final step is the assembly of the two panels, including injection of a liquid crystal material between the two panels to form the liquid crystal panel.
One of the most critical steps in manufacturing the color filter is the preparation of the black matrix pattern. The definition or sharpness of the edge of the black matrix is extremely important for several reasons. Unlike the colored ink cells, any variation in the black matrix edge, for example, caused by printing flow or unclean doctoring, is readily discemable when the final product is inspected. The color pixel edge, on the other hand, is typically hidden by the black matrix pattern. Consequently, to a certain extent, the black matrix hides imperfections in the color pixel edge, while there is nothing to hide imperfections in the black matrix.
The edge definition of the black matrix also affects the registration of the black matrix pattern with the thin film transistors or diodes located on the separate (rear) glass panel. Registration of the color pixels is also important, but to a lesser degree, because, the width of the black matrix pattern hides the transition area between individual color pixels and, thus, provides some leeway in registering the color pixels.
Unfortunately, while some printing techniques have successfully been employed to produce the color ink dots which make up the color pixels, the drive to achieve thinner (and thus higher resolution) black matrix lines has pushed the resolution capabilities of conventional printing techniques to their limit, because it becomes extremely difficult to maintain the required sharp edge definition using printing techniques. One of the problems associated with ink printing techniques arises from the inks' surface tension which tends to produce grid lines having rounded cross-sectional shapes.
The tendency of inks to produce grid lines having rounded cross-sectional shapes was addressed in U.S. Pat. No. 5,514,503 to Evans et al ("Evans"). However, while a significant advance, the Evans process has several shortcomings. In particular, the inks employed in Evans fail to doctor clearly resulting in black ink being left in the colored pixel area. In addition to being visually discemable, the black residue in the pixel area makes registration between the black matrix pattern and the thin film transistors or diodes difficult. In other imaging technologies, such as gravure printing, doctoring characteristics are controlled by the addition of solvents or mixtures of solvents having different evaporation rates. The thickness differences produces differences in the degree of solvent evaporation and, consequently, dryness between the etch and background. As a result, the etch and background have different degrees of tackiness which permits the selective transfer of only the material in the etch. However, the use of solvents in LCD manufacturing processes is undesirable because of the time it takes for the solvent to evaporate and because of the unreproducible and uncontrollable shrinkage which the inks experience upon evaporation of the solvent. Furthermore, inks which harden by solvent evaporation are a major contamination source, because the dried residue (for example on the background) tends to flake off. Therefore, conventional methods for adjusting an ink's doctoring characteristics are not applicable when using the preferred 100% solids ink.
For the above reasons, there is a need for an ink suitable for use in color filter display manufacture that doctors cleanly without the addition of solvent. Moreover, there is a need for fast-curing radiation curable formulations that are suitable for use in the preparation of ink formulations. The present invention is directed to meeting these needs.